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It’s an insurance for the chance that the council will come to you in the future and force you to fix it, and worst case, put it back to how it was originally. Pays put money to cover those costs if it happens.
Normally around £100-150 paid for by the seller, because the risk is normally pretty low for the above happening.
If it was high risk, or done very recently, the insurance might cost more, or not be able to be offered at all.
And if you do go ahead with purchase you may need indemnity insurance which the vendor should pay for (£150ish)